Many heavy duty on-off road vehicles include an auxiliary transmission in addition to the main transmission. Two related problems are associated with auxiliary transmissions that are used in combination with automatic main transmissions. First, because a vehicle equipped with an automatic main transmission does not have a clutch between major drive-line components, the operator must stop the vehicle in order to shift the auxiliary transmission from low to high or from high to low. Second, because the operator must stop the vehicle in order to shift the auxiliary transmission, an auxiliary transmission is not considered ideal with an automatic main transmission in many applications.
For example, if the operator desires to switch from high to low while ascending a steep, muddy grade, the vehicle may very well become immobile in the mud while the operator is shifting. If the vehicle is an emergency vehicle such as an aircraft rescue and fire fighting vehicle (ARFF), it may be dangerous for the operator to completely stop the vehicle to shift the auxiliary transmission. Finally, for obvious reasons, it may be very undesirable for the operator of a military vehicle to completely stop the vehicle in order to shift the auxiliary transmission, whether in a convoy or battlefield situation.
Auxiliary transmissions are often used with heavy equipment vehicles used in both on-road and off-road conditions. While auxiliary transmissions are supplied in heavy trucks, they have also been implemented in recreational off-road vehicles having four-wheel drive. Specifically, the low gear of an auxiliary transmission is especially useful when the recreational vehicle is towing a boat or a trailer up a steep hill or in muddy conditions. Further, the low gear is an important safety device when descending down steep hills in both on-road and off-road situations.
Two-speed auxiliary transmissions include low and high gears. Other auxiliary transmissions may include more than two gears. When the auxiliary transmission is in low gear, the top speed of the vehicle is dependent on the gear ratio and is normally limited to about 20 miles per hour even though the main transmission is in high gear.
Thus, there is a need for an improved shifting system for auxiliary transmissions. The shifting system must be able to shift the auxiliary transmission without use of a clutch so it may be used in combination with a fully automatic main transmission. The system should be applicable to all vehicles employing auxiliary transmissions with multi-speed gearcases and vehicles with or without fully automatic main transmissions.